Ink jet inks must have a very low viscosity, typically less than about 20 centipoise at the jetting temperature. While hot melt inks have been used, liquid inks are generally more suited to high volume industrial printing. One way to achieve this low viscosity is by including a substantial amount of organic liquids. In general, ink containing a substantial amount of organic liquids would produce undesirable emissions during the printing process. Such emissions are substantially avoided, however, with energy curable inks. Energy curable inks use low viscosity reactive materials to attain the desired viscosity. The reactive materials have reactive groups that are cured after printing with radiation, such as UV radiation or electron beams.
The low viscosity reactive materials in energy curable inks may include monoethylenically and polyethylenically unsaturated monomers and oligomers. The energy curable inks may also include unreactive oligomers and polymers. Because monofunctional monomers are particularly low in viscosity, ink jet inks to date have included substantial amounts of monofunctional monomers. On the other hand, monofunctional monomers are generally the most volatile of the reactive materials that might be included, resulting in some evaporation and undesirable emissions during the printing process. Further, polyfunctional monomers are desirable because they cure faster. Monofunctional monomers may lead to insufficient cure through chain termination. It would, therefore, be desirable to omit or limit the amount of such monofunctional monomers in the ink jet ink.
Ink jet inks containing pigments require more diluent monomer than do ink jet inks containing dyes. Ink jet inks containing pigments are more desirable, however, because pigments are more lightfast. While dyes are liquid or solids that can readily be dissolved in an ink jet ink, pigments are not so easily incorporated. Pigments must first be dispersed in a dispersing material, both to promote stability of the pigment particle in the ink and also to permit proper color development. For instance, U.S. Pat. No. 6,300,388 describes dispersing pigments in polyester, polyurethane, or polyacrylate polymers, especially in the form of high molecular weight block copolymers. Such polymers may be used in amounts from 2.5 to 100% by weight of the pigment. Thus, substantial amounts of such polymers may be brought in with the pigment dispersion, especially in the case of hard-to-disperse pigments. A substantial amount of monofunctional monomer is then included in making inks with such pigment dispersions to reduce the viscosity to that needed for ink jet printing. Further, it is undesirable to introduce non-reactive polymers into the UV curable ink, as they may slow the cure response and produce print with poorer durability and solvent resistance.
Thus, it would be desirable to prepare a UV-curable ink jet ink containing pigment without dispersing the pigment in a high viscosity material or in a material that is not UV curable.